Author:

David Yager-Elorriaga(University of Michigan)

This paper gives an in-depth experimental study of helical features on
magnetized, ultrathin foil-plasmas driven by the 1-MA linear transformer
driver at University of Michigan. Three types of cylindrical liner loads
were designed to produce: (a) pure magneto-hydrodynamic (MHD) modes (defined
as being void of the acceleration-driven magneto-Rayleigh-Taylor
instability, MRT) using a non-imploding geometry, (b) pure kink modes using
a non-imploding, kink-seeded geometry, and (c) MRT-MHD coupled modes in an
unseeded, imploding geometry. For each configuration, we applied relatively
small axial magnetic fields of $B_{z} =$ 0.2-2.0 T (compared to peak
azimuthal fields of 30-40 T). The resulting liner-plasmas and instabilities
were imaged using 12-frame laser shadowgraphy and visible self-emission on a
fast framing camera. The azimuthal mode number was carefully identified with
a tracking algorithm of self-emission minima. Our experiments show that the
helical structures are a manifestation of discrete eigenmodes. The pitch
angle of the helix is simply $m/kR$, from implosion to explosion, where $m$,
$k$, and $R$ are the azimuthal mode number, axial wavenumber, and radius of
the helical instability. Thus, the pitch angle increases (decreases) during
implosion (explosion) as $R$ becomes smaller (larger). We found that there
are one, or at most two, discrete helical modes that arise for magnetized
liners, with no apparent threshold on the applied $B_{z} $ for the
appearance of helical modes; increasing the axial magnetic field from zero
to 0.5 T changes the relative weight between the $m=0$ and $m=1$ modes.
Further increasing the applied axial magnetic fields yield higher $m$ modes.
Finally, the seeded kink instability overwhelms the intrinsic instability
modes of the plasma. These results are corroborated with our analytic theory
on the effects of radial acceleration on the classical sausage, kink, and
higher $m$ modes. \\
\par*Work supported by US DOE award DE-SC0012328, Sandia National Laboratories,
and the National Science Foundation. D.Y.E. was supported by NSF fellowship
grant number DGE 1256260. The fast framing camera was supported by a DURIP,
AFOSR Grant FA9550-15-1-0419.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2017.DPP.GI3.5